diff --git a/delta.h b/delta.h
index 8a56ec07992c75..fad68cfc45f6f4 100644
--- a/delta.h
+++ b/delta.h
@@ -86,8 +86,11 @@ void *patch_delta(const void *src_buf, unsigned long src_size,
  * This must be called twice on the delta data buffer, first to get the
  * expected source buffer size, and again to get the target buffer size.
  */
-static inline unsigned long get_delta_hdr_size(const unsigned char **datap,
-					       const unsigned char *top)
+/*
+ * Size_t variant that doesn't truncate - use for >4GB objects on Windows.
+ */
+static inline size_t get_delta_hdr_size_sz(const unsigned char **datap,
+					   const unsigned char *top)
 {
 	const unsigned char *data = *datap;
 	size_t cmd, size = 0;
@@ -98,6 +101,13 @@ static inline unsigned long get_delta_hdr_size(const unsigned char **datap,
 		i += 7;
 	} while (cmd & 0x80 && data < top);
 	*datap = data;
+	return size;
+}
+
+static inline unsigned long get_delta_hdr_size(const unsigned char **datap,
+					       const unsigned char *top)
+{
+	size_t size = get_delta_hdr_size_sz(datap, top);
 	return cast_size_t_to_ulong(size);
 }
 
diff --git a/object-file.c b/object-file.c
index e55bf1bfff670f..0c823e5fc4adb6 100644
--- a/object-file.c
+++ b/object-file.c
@@ -1086,8 +1086,8 @@ int odb_source_loose_write_stream(struct odb_source *source,
 	} while (ret == Z_OK || ret == Z_BUF_ERROR);
 
 	if (stream.total_in != len + hdrlen)
-		die(_("write stream object %ld != %"PRIuMAX), stream.total_in,
-		    (uintmax_t)len + hdrlen);
+		die(_("write stream object %"PRIuMAX" != %"PRIuMAX),
+		    (uintmax_t)stream.total_in, (uintmax_t)len + hdrlen);
 
 	/*
 	 * Common steps for write_loose_object and stream_loose_object to
@@ -2108,6 +2108,7 @@ int odb_source_loose_read_object_stream(struct odb_read_stream **out,
 	struct object_info oi = OBJECT_INFO_INIT;
 	struct odb_loose_read_stream *st;
 	unsigned long mapsize;
+	unsigned long size_ul;
 	void *mapped;
 
 	mapped = odb_source_loose_map_object(source, oid, &mapsize);
@@ -2131,11 +2132,18 @@ int odb_source_loose_read_object_stream(struct odb_read_stream **out,
 		goto error;
 	}
 
-	oi.sizep = &st->base.size;
+	/*
+	 * object_info.sizep is unsigned long* (32-bit on Windows), but
+	 * st->base.size is size_t (64-bit). Use temporary variable.
+	 * Note: loose objects >4GB would still truncate here, but such
+	 * large loose objects are uncommon (they'd normally be packed).
+	 */
+	oi.sizep = &size_ul;
 	oi.typep = &st->base.type;
 
 	if (parse_loose_header(st->hdr, &oi) < 0 || st->base.type < 0)
 		goto error;
+	st->base.size = size_ul;
 
 	st->mapped = mapped;
 	st->mapsize = mapsize;
diff --git a/odb/streaming.c b/odb/streaming.c
index 4a4474f891a07f..bd460f9adc2cb2 100644
--- a/odb/streaming.c
+++ b/odb/streaming.c
@@ -158,15 +158,26 @@ static int open_istream_incore(struct odb_read_stream **out,
 		.base.read = read_istream_incore,
 	};
 	struct odb_incore_read_stream *st;
+	unsigned long size_ul;
 	int ret;
 
 	oi.typep = &stream.base.type;
-	oi.sizep = &stream.base.size;
+	/*
+	 * object_info.sizep is unsigned long* (32-bit on Windows), but
+	 * stream.base.size is size_t (64-bit). We use a temporary variable
+	 * because the types are incompatible. Note: this path still truncates
+	 * for >4GB objects, but large objects should use pack streaming
+	 * (packfile_store_read_object_stream) which handles size_t properly.
+	 * This incore fallback is only used for small objects or when pack
+	 * streaming is unavailable.
+	 */
+	oi.sizep = &size_ul;
 	oi.contentp = (void **)&stream.buf;
 	ret = odb_read_object_info_extended(odb, oid, &oi,
 					    OBJECT_INFO_DIE_IF_CORRUPT);
 	if (ret)
 		return ret;
+	stream.base.size = size_ul;
 
 	CALLOC_ARRAY(st, 1);
 	*st = stream;
diff --git a/odb/streaming.h b/odb/streaming.h
index c7861f7e13c606..517e2ea2d3f5c3 100644
--- a/odb/streaming.h
+++ b/odb/streaming.h
@@ -21,7 +21,7 @@ struct odb_read_stream {
 	odb_read_stream_close_fn close;
 	odb_read_stream_read_fn read;
 	enum object_type type;
-	unsigned long size; /* inflated size of full object */
+	size_t size; /* inflated size of full object */
 };
 
 /*
diff --git a/packfile.c b/packfile.c
index 402c3b5dc73131..e198c9793f089f 100644
--- a/packfile.c
+++ b/packfile.c
@@ -1130,8 +1130,8 @@ unsigned long repo_approximate_object_count(struct repository *r)
 	return r->objects->approximate_object_count;
 }
 
-unsigned long unpack_object_header_buffer(const unsigned char *buf,
-		unsigned long len, enum object_type *type, unsigned long *sizep)
+static unsigned long unpack_object_header_buffer_internal(const unsigned char *buf,
+		unsigned long len, enum object_type *type, size_t *sizep)
 {
 	unsigned shift;
 	size_t size, c;
@@ -1142,7 +1142,11 @@ unsigned long unpack_object_header_buffer(const unsigned char *buf,
 	size = c & 15;
 	shift = 4;
 	while (c & 0x80) {
-		if (len <= used || (bitsizeof(long) - 7) < shift) {
+		/*
+		 * Each continuation byte adds 7 bits. Ensure shift won't
+		 * overflow size_t (use size_t not long for 64-bit on Windows).
+		 */
+		if (len <= used || (bitsizeof(size_t) - 7) < shift) {
 			error("bad object header");
 			size = used = 0;
 			break;
@@ -1151,13 +1155,25 @@ unsigned long unpack_object_header_buffer(const unsigned char *buf,
 		size = st_add(size, st_left_shift(c & 0x7f, shift));
 		shift += 7;
 	}
+	*sizep = size;
+	return used;
+}
+
+unsigned long unpack_object_header_buffer(const unsigned char *buf,
+		unsigned long len, enum object_type *type, unsigned long *sizep)
+{
+	size_t size;
+	unsigned long used = unpack_object_header_buffer_internal(buf, len, type, &size);
 	*sizep = cast_size_t_to_ulong(size);
 	return used;
 }
 
-unsigned long get_size_from_delta(struct packed_git *p,
-				  struct pack_window **w_curs,
-				  off_t curpos)
+/*
+ * Size_t variant for >4GB delta results on Windows.
+ */
+static size_t get_size_from_delta_sz(struct packed_git *p,
+				     struct pack_window **w_curs,
+				     off_t curpos)
 {
 	const unsigned char *data;
 	unsigned char delta_head[20], *in;
@@ -1204,10 +1220,44 @@ unsigned long get_size_from_delta(struct packed_git *p,
 	data = delta_head;
 
 	/* ignore base size */
-	get_delta_hdr_size(&data, delta_head+sizeof(delta_head));
+	get_delta_hdr_size_sz(&data, delta_head+sizeof(delta_head));
 
 	/* Read the result size */
-	return get_delta_hdr_size(&data, delta_head+sizeof(delta_head));
+	return get_delta_hdr_size_sz(&data, delta_head+sizeof(delta_head));
+}
+
+unsigned long get_size_from_delta(struct packed_git *p,
+				  struct pack_window **w_curs,
+				  off_t curpos)
+{
+	size_t size = get_size_from_delta_sz(p, w_curs, curpos);
+	return cast_size_t_to_ulong(size);
+}
+
+/*
+ * Like unpack_object_header(), but returns size via size_t* instead of
+ * unsigned long*. This is needed for >4GB objects on Windows where
+ * unsigned long is 32-bit but size_t is 64-bit. Used by streaming code
+ * to get the correct untruncated object size.
+ */
+static int unpack_object_header_sz(struct packed_git *p,
+				   struct pack_window **w_curs,
+				   off_t *curpos,
+				   size_t *sizep)
+{
+	unsigned char *base;
+	unsigned long left;
+	unsigned long used;
+	enum object_type type;
+
+	base = use_pack(p, w_curs, *curpos, &left);
+	used = unpack_object_header_buffer_internal(base, left, &type, sizep);
+	if (!used) {
+		type = OBJ_BAD;
+	} else
+		*curpos += used;
+
+	return type;
 }
 
 int unpack_object_header(struct packed_git *p,
@@ -1582,7 +1632,7 @@ int packed_object_info(struct packed_git *p,
 		       off_t obj_offset, struct object_info *oi)
 {
 	struct pack_window *w_curs = NULL;
-	unsigned long size;
+	size_t size;
 	off_t curpos = obj_offset;
 	enum object_type type = OBJ_NONE;
 	int ret;
@@ -1597,7 +1647,8 @@ int packed_object_info(struct packed_git *p,
 		if (!*oi->contentp)
 			type = OBJ_BAD;
 	} else if (oi->sizep || oi->typep || oi->delta_base_oid) {
-		type = unpack_object_header(p, &w_curs, &curpos, &size);
+		/* Use size_t variant to handle >4GB objects on Windows */
+		type = unpack_object_header_sz(p, &w_curs, &curpos, &size);
 	}
 
 	if (!oi->contentp && oi->sizep) {
@@ -1609,14 +1660,18 @@ int packed_object_info(struct packed_git *p,
 				ret = -1;
 				goto out;
 			}
-			*oi->sizep = get_size_from_delta(p, &w_curs, tmp_pos);
-			if (*oi->sizep == 0) {
+			/*
+			 * Use size_t variant to avoid die() on >4GB deltas.
+			 * oi->sizep is unsigned long, so truncation may occur,
+			 * but streaming code uses its own size_t tracking.
+			 */
+			size = get_size_from_delta_sz(p, &w_curs, tmp_pos);
+			if (size == 0) {
 				ret = -1;
 				goto out;
 			}
-		} else {
-			*oi->sizep = size;
 		}
+		*oi->sizep = (unsigned long)size;
 	}
 
 	if (oi->disk_sizep) {
@@ -2561,21 +2616,29 @@ int packfile_store_read_object_stream(struct odb_read_stream **out,
 	struct pack_window *window = NULL;
 	struct object_info oi = OBJECT_INFO_INIT;
 	enum object_type in_pack_type;
-	unsigned long size;
-
-	oi.sizep = &size;
+	size_t size;
 
+	/*
+	 * We need to check if this is a delta or if the object is smaller
+	 * than the big file threshold. For the initial check, we don't need
+	 * the exact size, just whether it qualifies for streaming.
+	 */
 	if (packfile_store_read_object_info(store, oid, &oi, 0) ||
 	    oi.u.packed.type == PACKED_OBJECT_TYPE_REF_DELTA ||
-	    oi.u.packed.type == PACKED_OBJECT_TYPE_OFS_DELTA ||
-	    repo_settings_get_big_file_threshold(store->source->odb->repo) >= size)
+	    oi.u.packed.type == PACKED_OBJECT_TYPE_OFS_DELTA)
 		return -1;
 
-	in_pack_type = unpack_object_header(oi.u.packed.pack,
-					    &window,
-					    &oi.u.packed.offset,
-					    &size);
+	/* Read the actual size using size_t to handle >4GB objects on Windows */
+	in_pack_type = unpack_object_header_sz(oi.u.packed.pack,
+					       &window,
+					       &oi.u.packed.offset,
+					       &size);
 	unuse_pack(&window);
+
+	/* Now check the big file threshold with the correct size */
+	if (repo_settings_get_big_file_threshold(store->source->odb->repo) >= size)
+		return -1;
+
 	switch (in_pack_type) {
 	default:
 		return -1; /* we do not do deltas for now */